1. Field of the Invention
The invention relates to a process for producing highly doped semiconductor wafers, in which at least two dopants are used. The invention also relates to dislocation-free, highly doped semiconductor wafers.
2. Background Art
Highly doped semiconductor substrates with a low resistivity are needed, for example, for the fabrication of power components. In conventional processes, in each case, only one specific element is incorporated as a dopant in the semiconductor wafers in order to reduce the substrate resistance, for example, B, P, As, or Sb in silicon. Typical dopant concentrations for highly doped silicon are greater than 1018/cm3, and are usually achieved by adding the dopant to the melt before or during crystal growth or through diffusion of the dopant into the semiconductor wafers which have been processed from the single crystal that has been grown.
The diffusion constant of the dopant in the semiconductor material is highly important. Since the deposition of epitaxial layers and the processing of semiconductor wafers are usually carried out at high temperatures, dopant can diffuse from the substrate into the epitaxial layer. When dopants with a high diffusion constant are used, therefore, the transition range between substrate and epitaxial layer is wider than when dopants with a low diffusion coefficient are used. The result is that when using dopants with high diffusion coefficients, thicker epitaxial layers are required, up until complete transition of the materials properties. In industrial fabrication of semiconductor elements this leads to higher production costs. A further drawback of dopants with high diffusion coefficients results from the phenomenon known as auto-doping during the deposition of epitaxial layers. In this phenomenon, the dopant passes out of the substrate via the gas phase into the epitaxial layer and unintentionally alters the resistivity thereof. Therefore, it is preferable to use dopants which have a low diffusion coefficient in the semiconductor material.
However, the use of large quantities of dopant in the production of single crystals has various associated problems which restrict the minimum substrate resistances that can be achieved: high dopant concentrations in the melt can lead to precipitations of the dopant in the melt and to constitutional supercooling, which in both cases prevents single-crystalline growth. Moreover, large quantities of individual dopants may evaporate out of the melt, which increases the quantity of dopant required and can lead to the undesirable formation of toxic chemical compounds. A further problem with high dopant concentrations is that above certain concentrations some of the dopant atoms incorporated in the semiconductor material may be electrically inactive. This occurs, for example, if silicon is so strongly doped with arsenic that the resistivity drops below approximately 5 mOhm·cm (Quick Reference Manual for Silicon Integrated Circuit Technology; W. E. Beadle, J. C. C. Tsai & R. D. Plummer; John Wiley & Sons, New York, Chichester, Brisbane; pp. 2-70, 1985). On account of the abovementioned effects, dislocation-free crystal growth is only possible down to a certain minimum resistance, with the dopant limit concentration being determined by the procedure used in crystal growth and by the type of dopant.